Over the last decade, nanoparticles, i.e. particles having sizes below 1 micrometer, have attracted a great deal of interest in research and industry due to their unique properties. Research and development in the optoelectronic area have focused on luminescent particles in view of their possible application in light emitting diodes (LED), displays, optoelectronic devices in nanometer dimensions or as a light source in low threshold lasers.
Among luminescent materials, a distinction is often made between semiconductor and non semiconductor materials.
Semiconductor nanoparticles, such as II-VI or III-V semiconductors which may be doped or not, are characterized by a quantum confinement of both the electron and hole in all three dimensions which leads to an increase in the effective band gap of the material with decreasing crystalline size. Consequently, it is possible to shift both the optical absorption and emission of semiconductor nanoparticles to the blue (higher energies) as the size of the nanoparticles gets smaller. However, the size dependency of the emission is undesired for many applications since it requires a very strict control of the particle size distribution or size selection steps before industrial applications can be envisaged.
Water-soluble core/shell semiconductor nanocrystals are, for instance, described in WO 00/17655.
In contrast thereto, it constitutes the particular attractivity of nanocrystalline non-semiconductor-based luminescent materials, in particular, lanthanide-doped metal oxides or salts, that their fluorescent emission is relatively narrow and does not depend to a greater extent on the host material and the size of the nanoparticles. It is rather only the type of lanthanide metal which determines the emission color. PCT/DE 01/03433 assigned to the same applicants discloses a generally applicable synthesis method for lanthanide-doped nanoparticles of this type. These nanoparticles can be produced in sizes (below 30 nm) no longer interacting with the wavelength of visible light, thereby leading to transparent dispersions e.g. in organic or aqueous solvents.
One important parameter governing the usefulness of lanthanide-doped nanoparticles is their quantum yield. As quantum yield, we understand the ratio of photons emitted to that absorbed.
As a rule, it is desired to reach with nanocrystalline materials quantum yields in the same order as for the corresponding macrocrystalline luminescent material. The commercially available macrocrystalline green luminescing phosphor (La0.45, Ce0.40)PO4:Tb0.15 shows for instance total quantum yields in the order of 93% (including the UV emission).
However, due to the much higher surface/volume ratio of nanocrystalline materials, the likelihood of surface luminescence quenching phenomena increases.
K. Riwotzki et al., J. Phys. Chem. B 2000, 104, 2824-2828, “Liquid phase synthesis doped nanoparticles: colloids of luminescent LaPO4:Eu and CePO4:Tb particles with a narrow particle size distribution”, report for instance quantum yields of less than 10% for LaPO4:Eu upon excitation at 277 nm and CePO4:Tb quantum yields of 16%, if the emission of cerium is included, and 11% if the emission of terbium is considered only. The observed values are far removed from the theoretical quantum yields of 89% and 38% calculated for nanocrystalline LaPO4:Eu and CePO4:Tb, respectively. The authors of this article assume that the excited state of the host is depleted by energy transfer not only to the luminescent centers but also to centers where radiationless recombination occurs. Likely centers for the radiationless combination may be the same quencher ions to which energy from the luminescing ions is transferred or may be surface states of the nanoparticles. In this context the authors mention that growing a shell of inert material around each nanoparticle has already been successfully applied to semiconductor nanoparticles thereby increasing their luminescence quantum yields to values between 30% and more than 60% (references 37-44 of K. Riwotzki et al).
Jan W. Stouwdam and Frank C. J. M. van Veggel, Nano Letters, ASAP article, web release May 15, 2002, “Near-infrared emission of redispersible Er3+, Nd3+ and Ho3+ doped LaF3 nanoparticles” discloses the preparation of nanoparticles which may be promising materials for polymer-based optical components, because they show luminescence and wavelengths between 1300 and 1600 nm where silicon-based optical fibres have their maximum transparency. The authors measured a bi-exponential decay for the luminescence of these nanoparticles and speculate in this context that a way to improve the luminescence of ions located at or near the surface might be to grow a layer of undoped LaF3 around the particles.
K. Riwotzki et al., J. Phys. Chem. B 2001, 105, 12709-12713, “Colloidal YVO4:Eu and YP0.95V0.05O4:Eu particles: Luminescence and Energy transfer process” discuss a YPO4 coating for YVO4:Eu cores as conceivable means to improve the low quantum yield of 15% observed for nanocrystalline YVO4:Eu.
The synthesis of core/shell particles, however, encounters major obstacles. Firstly, an independent growth of the starting materials used for the shell is to be prevented since the same would lead to co-existing homogenous nanoparticles of different composition. Simultaneously, exchange processes between the individual nanoparticles and Oswald ripening have to be suppressed. Oswald ripening is a phenomenon occurring in dispersions of small particles at higher temperatures and involving the growth of bigger particles at the expense of smaller particles. This mechanism also leads to the randomisation of the different compositions used for core and shell.
M. Haase et al, Journal of Alloys and Compounds, 303-304 (2000) 191-197, “Synthesis and properties of colloidal lanthanide-doped nanocrystals” compare the properties of lanthanide-doped nanocrystals that were prepared hydrothermally (in aqueous solution) and in high-boiling organic solvents (tributylphosphat), respectively. The authors report a luminescence quantum yield of 15% at room temperature for YVO4:Eu nanoparticles.
Luminescence quantum yields in the same order (15%) are also mentioned in G. A. Hebbink et al, Advanced Materials 2002, 14, No. 16, pages 1147-1150, “Lanthanide(III)-doped nanoparticles that emit in the near-infrared”, for Nd3+ and Er3+-doped LaPO4 particles.
Therefore, up to date, non-semiconductor core/shell particles have not yet been synthesized.
Accordingly, it is an object of the present application to provide a synthesis for specific non-semiconductor core/shell particles.
Further, the present application aims at increasing the quantum yield of homogenous luminescent non-semiconductor particles.